A force sensor having a function to output, as an electrical signal, force acting in a predetermined axial direction and torque acting around a predetermined axis of rotation is for example described in Patent Literature 1 and is widely used in force control of industrial robots. Such sensors have been employed in life support robots in recent years and thus high safety is desired. However, for example a force sensor of the current capacitance type includes a mechanical unit, a detection unit of capacitance (detection unit of force), and an electronic circuit including a microcomputer but may disadvantageously fail due to condensation, an impact, an overload, or contamination between a pair of parallel flat plates providing the capacitance.
As a simple method for determining whether a force sensor has failed, it is only required to align the multiple (e.g. three) force sensors described in Patent Literature 1 in parallel and to evaluate a difference of output signals of the force sensors. In this method, each two of the three output signals are compared and the force sensor is determined as normally functioning when the difference between the output signals of each two of the force sensors is within a predetermined range. On the other hand the force sensor is determined as not normally functioning (out of order) when the difference is not within the predetermined range.